Those who are in a serious relationship.How should I approach this matter!!Please Help!?

I am in a five year relationship. Lately we been getting into bickering arguments about small things.Things that shouldn't even be an issue. Also it seems that my significant other has been acting different towards me, by saying smart things and taking things defensively. We got into a argument last night, and he told me that he didn't feel like talking anymore,told me he loves me and bye(smartly),and just hung up on me.We never got off the phone like this.So my question is, how can I approach this to him.Thanks to all for suggestions.Take care.